As one kind of antenna used for communications using electromagnetic waves at frequencies in or higher than the VHF band, a sleeve antenna has been conventionally known. As the simplest structure of this sleeve antenna, a sleeve antenna 50 shown in FIG. 9 has been known.
For this sleeve antenna 50, a sleeve antenna has been known in which a radiating portion 14 is formed by leaving only a core conductor 20 including an insulating member 18 as an outer skin at an end portion of a coaxial feed line 12, which is referred to generally as a coaxial cable, and an outer conductor (braided wire) 30 formed of the braided wire 30 that had covered the radiating portion 14 is folded in a reverse direction (or the outside of the braided wire 30 is covered with a coaxial circular tube) to obtain a sleeve 10. Each of the length of the radiating portion 14 and the length of the sleeve 10 is generally set to a length of λ/4 where λ is the wavelength of a radio wave desired to be received, but a sleeve antenna including those having a length of λ/2 or λ/8, or another length has also been known.
Moreover, as an improvement to the sleeve antenna 50 having the simplest structure described above, a meander line sleeve antenna using a dielectric print and having two antenna elements that are small and provide high gain is disclosed (for example, refer to PTL 1).
Moreover, a wide-band and high-gain earphone antenna that reduces an influence on the human body is disclosed, and a portable radio apparatus that ensures stability of reception is also disclosed (for example, refer to PTLs 2 and 3).